An optical disk serving as an optical recording medium includes a transmissive substrate having a given thickness so that the transmissive substrate covers a recording surface of the optical disk in order to protect the recording surface. An optical pickup serving as an information reading section reads out recorded information from the optical disk based on an amount of light which is reflected off the recording surface when the recording surface is irradiated with reading beam light via the transmissive substrate.
Note, however, that in a case of producing optical disks, it is difficult to form transmissive substrates of all of the optical disks so that the transmissive substrates have thicknesses within a specified value, and this typically causes a thickness error of several μm. Such a thickness error of the transmissive substrate causes a spherical aberration. The occurrence of the spherical aberration causes a problem that an amplitude level of an information reading signal and/or a tracking error signal may be considerably decreases, and this therefore reduces accuracy of information reading. That is, in a case where an optical disk is replaced with another optical disk, a thickness of a transmissive substrate changes, and this changes a spherical aberration. Accuracy of information reading is therefore reduced in a case where no process is carried out with respect to the spherical aberration thus changed.
As such, in order to improve accuracy of information reproduction, it is necessary to correct the spherical aberration. For example, Patent Literature 1 discloses a technique of correcting such a spherical aberration. According to the technique disclosed in Patent Literature 1, the spherical aberration is corrected so that an amplitude of an RF (Radio Frequency) signal becomes maximum.